HPOS: Make sure billing and shipping address indexes are set on checkout

[james-allan]

Describe the bug

While testing #226 I discovered that the billing and presumably the shipping address indexes weren't being saved correctly on checkout.

Here's a comparison between trunk on the left and refactor-create-subscription on the right. Notice the billing address index only contains the email field.

To Reproduce

1. Purchase a subscription on the refactor-create-subscription branch.
   • no HPOS setup is required. Just using the wp post architecture is fine.
2. In the database look at the subscription and notice the billing address index is incomplete.

Expected behavior

The billing and shipping address indexes should be a concatenation of the all address fields.

Actual behavior

Product impact

• Does this issue affect WooCommerce Subscriptions? yes/no/tbc, add issue ref
• Does this issue affect WooCommerce Payments? yes/no/tbc, add issue ref

Additional context

This issue happens because of the wcs_copy_order_address() function which uses a legacy function set_address() to copy the address data. That function sets the address data directly into post meta which means that when the subscription is eventually saved, those billing fields aren't updated here which causes the indexes to not be updated.

We should replace the set_address() with modern APIs.